odb: stop splitting alternate in `odb_add_to_alternates_file()`

odb: stop splitting alternate in `odb_add_to_alternates_file()`

When calling `odb_add_to_alternates_file()` we know to add the newly
added source to the object database in case we have already loaded
alternates. This is done so that we can make its objects accessible
immediately without having to fully reload all alternates.

The way we do this though is to call `link_alt_odb_entries()`, which
adds _multiple_ sources to the object database source in case we have
newline-separated entries. This behaviour is not documented in the
function documentation of `odb_add_to_alternates_file()`, and all
callers only ever pass a single directory to it. It's thus entirely
surprising and a conceptual mismatch.

Fix this issue by directly calling `odb_add_alternate_recursively()`
instead.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>